1. Field of the Invention
The invention relates to a nasal mask assembly for use with non-invasive positive pressure ventilation (NIPPV) applied to small adults or pre-adults with sleep disordered breathing (SDB).
2. Description of Related Art
Sleep-disordered breathing is a general term for a sleep disorder with apneas and hypopneas. Apneas are generally taken to be a cessation of airflow for ten seconds or longer. Hypopnea are generally taken to be a 50% or greater decrease in air flow for ten seconds or longer. Both apneas and hypopneas cause sleep arousals—moments when an individual wakes enough to resume breathing but not enough to remember any interruption of sleep. Some arousals simply cause the sleeper to shift into a lighter stage of sleep. In either case, the arousal lessens the quality of sleep. Apneas and hypopneas may cause blood oxygen levels to drop. Apneas and hypopneas result from upper airway obstruction, either full or partial, or a dysfunction of the body's automatic drive to breathe.
Obstructive Sleep Apnea (OSA) is a common disorder. Estimates for the number of Americans with OSA vary depending on the criteria researchers use for the study. Conservative estimates, however, put the number of adult Americans with OSA at approximately 20 million. OSA occurs because of upper airway obstructions that can cause you to snore or to stop breathing. Obstructions occur during sleep for two primary reasons: lack of muscle tone and/or gravity. Excess tissue in the upper airway and anatomic abnormalities compound these factors. During sleep, especially in REM sleep, our bodies relax, and muscle tissues like the tongue and soft palate lose their slight rigidity. Because we tend to sleep lying down, gravity pulls these tissues toward the back of the throat and closes the upper airway.
The use of nasal Continuous Positive Airway Pressure (nasal CPAP) to treat obstructive sleep apnea (OSA) was taught by Sullivan in U.S. Pat. No. 4,944,310. Today apparatus for OSA typically comprises (i) a blower which provides a supply of air or breathable gas at positive pressure, (ii) an air delivery conduit connected to the blower, and (iii) a patient interface, such as a nasal mask, which is connected to the air delivery conduit.
A variety of nasal masks have been developed. One such mask is the MIRAGE® mask, manufactured by ResMed Limited and described in U.S. Pat. Nos. 6,112,746; 6,357,441; 6,119,693 and 6,463,931, amongst others. Another such mask is the ULTRA MIRAGE® mask, also manufactured by ResMed Limited. The ULTRA MIRAGE® mask is described in U.S. Pat. Nos. 6,112,746, 6,357,441, 6,374,826, 6,412,487, 6,439,230 and 6,463,931.
The American Academy of Pediatrics, in a Technical Report on the Diagnosis and Management of Childhood Obstructive Sleep Apnea Syndrome (OSAS) (PEDIATRICS Vol. 109 No. 4 April 2002.) noted that “Snoring is a common occurrence in childhood, with reported prevalence between 3.2% and 12.1%. The prevalence of childhood OSAS is difficult to estimate, largely because published studies use different PSG criteria for its ascertainment. Reports range from 0.7% to 10.3%.”
A key factor in the efficacy of therapy and compliance of patients with therapy is the comfort and fit of masks. Whilst there are a large number of masks designed for adults, there are relatively few designed to suit children.
One mask designed for children is the SULLIVAN® Nasal CPAP system, Infant Nasal Bubble Mask System with Sensor tubing, manufactured by ResMed Limited. See FIG. 1. In addition, Respironics Inc. manufactures a “Comfort Flap Small Child” product.
Other infant masks are shown in the following patents: FR 2775905; GB 2277688; US 2002/0104531; U.S. Pat. Nos. 3,827,433; 4,232,476; 4,406,283; 4,774,946; 4,896,666; 4,832,015; 5,271,391; 5,318,590; 5,462,050; 5,509,408; 5,535,741; 5,813,423; 5,660,174; 6,418,929; WO 01/32250; WO 02/05883; WO 95/09023
One adult mask is the VISTA™ mask, manufactured by ResMed Limited, and described in Australian Provisional Patent Application PS1926, filed 23 Apr. 2002; U.S. 60/377,254 filed 03 May 2002; U.S. 60/397,195 filed 22 Jul. 2002; U.S. 60/402,509 filed 12 Aug. 2002; and U.S. Ser. No. 10/391,440, 10/390,682, 10/390,681, 10/390,720 and 10/390,826, all filed 19 Mar. 2003, the contents of which are hereby incorporated by cross-reference. An ornamental design applied to the VISTA™ mask is shown in design patent application US Des 29/166,190, the contents of which are hereby incorporated by reference.
The VISTA™ mask includes a cushion, a frame, an elbow and headgear including a yoke.
While the VISTA™ mask is particularly suitable for adults, the problem arises as to how such an adult mask can be adapted to suit children.
International Patent Application WO2001/32250 (Sullivan & Wilkie) describes a mask for supplying gas under pressure to the nasal airway of an infant human. In that application, the following is stated:
“ . . . until now, infant masks have been developed on the basis of scaling down the adult mask to approximate to the infantface and nose. The problems with this scaling down process are threefold.
    First, the adult nose and middle third of the face is very different in shape from that of the infant. The adult nose is more elongated than, and protrudes far more from the surface of the face compared to the infant nose which is relatively flat with no bridge, with the nares (nostril passages) pointing outwards. Therefore in order to fit the adult nose the base of the mask has a triangular shape elongated in the vertical axis. In contrast, with an infant, the width at the base of the nose approximates the height from the base of the nose (nares) to the apex of the nose (nasion). The proportional shape of the nasal area of an adult is rectangular compared with a square proportional shape for an infant. In addition to this basic difference in proportional shape, the adult face has quite marked contours especially around the nose and cheek area which are absent in the infant. The adult mask must therefore have acute angles which accommodate these facial contours. Thus, when an adult mask is scaled down for an infant, not only are the proportions wrong for the infant nose and face, but the angles which are unnecessarily incorporated, inadvertently introduce a new problem. Because the infant has a relatively flat nose, and virtually no bridge, the angles promote formation of channels in the sealing margin of the mask, especially in the region of the nasal bridge.    Secondly in adult mask designs, the straps of the head harness connect with lugs on the rigid manifold in the order of 20 mm away from the surface of the face to allow the mask to accommodate the height of the adult nose. Because of this a potential fulcrum effect is created. In the adult this fulcrum effect is not as problematic as in the infant, not only because the adult is less mobile during sleep . . . but also because the contours of the adult face and cheeks can offset this rise. In the infant, when the mask used is merely a scaled down adult mask, the elevation of the straps lugs above the face is about 12 mm. This by itself creates a potential fulcrum as it does on the adult but the effect is enhanced by the fact that there is no offset from the infant cheek due to the smaller facial area. Consequently, the straps holding the mask in place come into contact with the side of the face in the infant, compared to the cheek in the adult.    Thirdly, because the attachment of the paediatric mask to the face and head mimics that of the adult mask, the torsional forces are increased. The greater torsional effect is due to the decreased surface area of the mask face contact relative to the air delivery pipe. Thus relatively minor movements can result in sufficient torsional forces to cause movement at the interface between the mask and the infants face.”